Add subdomain offset setting

Add a setting "subdomain offset" for the app, which can be used to
change the behavior of req.subdomains. This is useful when our "base"
domain contains more than two parts, e.g. example.co.uk, and also
when we are running locally with domains like xxx.local.

The default behavior is still to return all but the last two parts.
Merge pull request #1475 from gmethvin/subdomain_offset

Add subdomain offset setting